Helpful information in logfiles generated by logging plugin

In our company we (want to) use the log file generated by pytest --log-file logfile.txt for debugging issues with our tests in our CI. We use a log
file because even the failed tests may contain a lot of log messages, that
would spam the console output of pytest.

If there are some failing tests, the exceptions are unfortunately not part
of the log file, because the logging module doesn't create logging message
on it's own.

It would be helpful for debugging purposes, if the generated log file
contained not only the traceback (as is output to stdout by the pytest
frontend) but also separator lines, like the ones used for the live logging
output, e.g. --- live log setup ---.

Start by reproducing the issue with pytest --log-file logfile.txt and a failing test, then compare the generated file with the live logging output. Trace the logging plugin entry point and existing tests for log-file and live-log behavior. Done means the file includes separator lines such as the setup section and the failure traceback without flooding console output.